Editing Crohn's disease (section)

== '''Aetiology is unknown, but the following causes have been proposed. The true aetiology is likely a combination.''' ==

* Infectious
** Mycobacterial infections are plausibly causative, particularly ''Mycobacterium paratuberculosis'' and enteroadherent ''E. coli''
** PCR studies have confirmed the presence of mycobacteria in patients with CD
** Transplantation of tissue from CD patients has resulted in ileitis, but antimicrobial therapy against mycobacteria has not cured established CD
* Immunologic
** Humoral and cell-mediated immune reactions directed against intestinal cells in CD suggest an autoimmune phenomenon
** Especially IL-1, IL-2, IL-8 and TNF-a
** Remains controversial - may reflect effect rather than cause
* Genetic
** >200 alleles associated with CD, most frequently NOD2, MHC, and MST1 3p21
** NOD2 is associated with decreased expression of antimicrobial peptides by Paneth cells (homozygosity confers a 17-40x risk)
** Family history is the strongest single risk factor
** Twin genetic comparisons have suggested that simple Mendelian inheritance does not account for occurrence
**
* Other factors thought to be possible but less important:
** Environmental
*** As countries adopt a more Western lifestyle, the incidence of CD rises
*** Dysbiosis - decrease in intraluminal ''Bacteroides'' and ''Firmicutes'' and increase in ''Gammaproteobacteria'' and ''Actinobacteria'' are associated with higher risk
** Dietary
*** Decreased fibre
*** Increased fat
** Smoking
** Psychosocial
** Medications
*** COCP, aspirin, NSAIDs